Knowledge of the three-dimensional structures of proteins and multi-protein assemblies is of great importance to the understanding of fundamental biological processes and the design of more effective Pharmaceuticals. For many such species, which cannot be crystallized and are too large for NMR analysis, electron microscopy (EM) is the only technique that can provide useful structural information. The attachment of electron-dense molecules, containing clusters of heavy metallic elements that scatter electrons with high efficiency, is often necessary to EM data collection and analysis. Practitioners of high-resolution EM usually have access to only two electron-desnse materials and a limited set of attachment methods available from commercial suppliers. In many cases, these resources fail and the target must be either abandoned or laboriously re-engineered. We propose to create new electron-dense compounds and develop methods to rapidly identify the best ways to connect them to target proteins. The goal is to give the EM community an expanded set of materials and techniques for protein labeling, so that a greater fraction of targets can be structurally characterized. The effort will be comprised of equal parts chemical synthesis, chemical analysis to measure the overall efficiency of attachment reactions, and rigorous EM testing on a versatile virus particle platform. Successful methods will then be applied to important protein systems that have thus far resisted structural characterization by electron microscopy.